In a coffee shop, you still have the option of not using the connection at all if it is unencrypted, and your OS should notify you that its unsafe (I know WinXP does this, though I don't have much experience with linux or osx). So, while you don't have the option of using a wired signal, you're still not forced to use an insecure connection.
As for the example, I think in the interest of making it more realistic, I lost a lot of the impact. Regardless, you sort of made my point for me. We're talking about the expectation of privacy, and the moment you start broadcasting your private information - whether shouting in public or sending out an unencrypted data signal - it can no longer be considered private.
As for your example, it depends entirely on the letter/package. Opening someone elses mail is a felony/indictable offence, so there is a reasonable expectation of privacy. AFAIK, while connecting to someone elses private connection might be illegal (war driving, etc.), I don't think that's what Google was doing. Please correct me if I'm wrong, but it seems to be that Google was just sniffing data in the air rather than making an actual connection to the network.
What Google seems to have done is more like reading a postcard. The data was clearly visible, you just needed to know how to read it. I don't think anyone could rationally say that postcards are a secure form of communication.